Zeolites are hydrated alkali metal alumino silicates which are both naturally occurring and synthetically produced. The synthetic zeolites have compositions similar to the natural zeolites and the most common of these zeolites are chemically, sodium alumino silicates. Zeolites consist basically of a three-dimensional framework of SiO.sub.4 and AlO.sub.4 tetrahedra which are crosslinked by the sharing of oxygen atoms so that the ratio of oxygen atoms to the total of aluminum and silicon atoms is equal to 2 or 0/(Al+Si)=2. The electrovalence of each tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation such as a sodium ion. The spaces between the tetrahedra are occupied by water molecules prior to dehydration.
Zeolites are classified on the basis of their adsorption characteristics particularly on the basis of the size and shape of the adsorbate molecule. Thus they are often called molecular sieves. Molecular sieves have adsorption areas on the interior of a large number of uniformly sized pores of molecular dimensions. With such an arrangement molecules of a certain size and shape enter the pores and are adsorbed while larger or differently shaped molecules are excluded. Zeolites are generally identified by letter designation such as zeolite A, zeolite X or zeolite Y and these materials are distinguished from other zeolites and silicates on the basis of their X-ray powder diffraction patterns and physical characteristics, including pore size opening. The basic formula for crystalline sodium zeolites may be represented as follows: EQU Na.sub.2 O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
In general a specific zeolite will have values for x and y that fall in a definite range. The value of x for a particular zeolite will vary somewhat since the aluminum atoms and the silicon atoms occupy essentially equivalent positions in the lattice. For zeolite A, an average value for x is about 1.85 with the x value falling within the range 1.85.+-.0.5. For zeolite X, the x value falls within the range 2.5.+-.0.5. Zeolites are well known in the art and are produced synthetically for example in accordance with the teachings of U.S. Pat. Nos. 2,882,243, 2,982,612 and 3,058,805 for zeolite A and 2,882,244 and 2,979,381 for zeolite X. References to the zeolites may of course be found in many other patents and publications.
Synthetic zeolites or sodium alumino silicates are prepared for the reaction of sodium silicate and sodium aluminate. When these reactants are mixed there is formed a reaction mixture which will contain water, sodium oxide, silica and alumina. A reaction of this type is disclosed in U.S. Pat. No. 4,041,135 to Williams et al, commonly assigned, in which process the reaction mixture includes molar ratios of these reactants as follows:
H.sub.2 O:Na.sub.2 O 35:1 to 200:1 PA0 Na.sub.2 O:SiO.sub.2 0.8:1 to 3:1 PA0 SiO.sub.2 :Al.sub.2 O.sub.3 0.5:1 to 2.5:1
The reaction is carried out at a temperature ranging from about 70.degree. to 180.degree. C. Normally in processes of this type, after completion of the reaction, the temperature of the mixture is then elevated to permit the intermediate amorphous product to crystallize to the desired zeolite product. Thereafter the solid zeolite is recovered by separation from the liquid or mother liquor, preferably by filtration. The resulting wet cake is then spray dried after washing to produce the final product.
It is a problem in separation of the zeolite product from its mother liquors to obtain a suitable level of purity of the product prior to drying. The present invention provides a method by which highly pure zeolites can be produced in the filtration process.